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Atmos. Meas. Tech., 9, 1947–1959, 2016
www.atmos-meas-tech.net/9/1947/2016/
doi:10.5194/amt-9-1947-2016
© Author(s) 2016. CC Attribution 3.0 License.
The performance and the characterization of laser
ablation aerosol particle time-of-flight
mass spectrometry (LAAP-ToF-MS)
Rachel Gemayel1, Stig Hellebust1,a, Brice Temime-Roussel1, Nathalie Hayeck1,b, Johannes T. Van Elteren2,
Henri Wortham1, and Sasho Gligorovski1
1Aix Marseille Université, CNRS, LCE UMR 7376, 13331, Marseille, France2National Institute of Chemistry, Slovenia, Laboratory for Analytical Chemistry, Hajdrihova 19, 1000 Ljubljana, Sloveniaanow at: Central Statistics Office, Cork, Irelandbnow at: Université de Lyon 1, CNRS, UMR 5256, IRCELYON, Institut de Recherches sur la Catalyse et
l’Environnement de Lyon, Villeurbanne, 69626, France
Correspondence to: Rachel Gemayel (rachel.gemayel@etu.univ-amu.fr)
and Sasho Gligorovski (saso.gligorovski@univ-amu.fr)
Received: 18 November 2015 – Published in Atmos. Meas. Tech. Discuss.: 20 January 2016
Revised: 6 April 2016 – Accepted: 13 April 2016 – Published: 2 May 2016
Abstract. Hyphenated laser ablation–mass spectrometry in-
struments have been recognized as useful analytical tools for
the detection and chemical characterization of aerosol parti-
cles. Here we describe the performances of a laser ablation
aerosol particle time-of-flight mass spectrometer (LAAP-
ToF-MS) which was designed for aerodynamic particle siz-
ing using two 405 nm scattering lasers and characterization
of the chemical composition of single aerosol particle via ab-
lation/ionization by a 193 nm excimer laser and detection in a
bipolar time-of-flight mass spectrometer with a mass resolv-
ing power of m/1m> 600.
We describe a laboratory based optimization strategy for
the development of an analytical methodology for charac-
terization of atmospheric particles using the LAAP-ToF-MS
instrument in combination with a particle generator, a differ-
ential mobility analyzer and an optical particle counter. We
investigated the influence of particle number concentration,
particle size and particle composition on the detection effi-
ciency. The detection efficiency is a product of the scattering
efficiency of the laser diodes and the ionization efficiency or
hit rate of the excimer laser. The scattering efficiency was
found to vary between 0.6 and 1.9 % with an average of
1.1 %; the relative standard deviation (RSD) was 17.0 %. The
hit rate exhibited good repeatability with an average value of
63 % and an RSD of 18 %. In addition to laboratory tests,
the LAAP-ToF-MS was used to sample ambient air during a
period of 6 days at the campus of Aix-Marseille University,
situated in the city center of Marseille, France. The optimized
LAAP-ToF-MS methodology enables high temporal resolu-
tion measurements of the chemical composition of ambient
particles, provides new insights into environmental science,
and a new investigative tool for atmospheric chemistry and
physics, aerosol science and health impact studies.
1 Introduction
Atmospheric aerosols, defined as an assembly of solid or liq-
uid particles suspended in a gas (Finlayson-Pitts and Pitts,
2000), have a large impact on human health (Dockery and
Pope, 2006) and global climate (Poeschl, 2005). Ambient
aerosols typically span a size range from 3 nm to 10 µm in di-
ameter. Between these particles, those with a diameter larger
than 5 µm are rapidly removed by gravitational settling while
aerosols with a diameter in the nanometer range, depend-
ing on the chemical composition and local meteorology, may
drift in the atmosphere for a prolonged period of time. Most
of the elements that are vaporized during various human ac-
tivities (e.g., coal combustion) tend to condense and form
fine particles with a high surface-to-volume ratio which can
Published by Copernicus Publications on behalf of the European Geosciences Union.
1948 R. Gemayel et al.: Laser ablation aerosol particle time-of-flight mass spectrometry
be transported over long distances (Canagaratna et al., 2007).
In addition, the smaller particles exhibit more adverse health
effects compared to the larger particles since they are more
likely to penetrate the human lung and even enter the blood
stream (Dockery and Pope, 2006). A recent study (Lelieveld
et al., 2015) has shown that outdoor air pollution leads to
3.3 million premature deaths per year worldwide, predomi-
nantly in Asia, mostly due to PM2.5 (particulate matter). This
figure could double by 2050 if emissions continue to rise at
the current rate.
A detailed understanding of the particle sizes and
the chemical composition of atmospheric particles is of
paramount importance to understand their impact on health
and climate. Hence, there is a need for the development of ap-
propriate analytical methods for on-line, time-resolved mea-
surements of atmospheric particles. In the last decade sev-
eral hyphenated laser ablation – mass spectrometry instru-
ments have been developed (see for instance Gaie-Levrel et
al. (2012) with the aim of chemically characterizing aerosol
particles. Murphy (2007) has reviewed the development and
implementation of single particle laser mass spectrometers.
These instruments appear promising for aerodynamic sizing
of particles and characterization of their chemical composi-
tion. The advantage of using laser ionization compared to
methods based on thermal desorption, such as that applied
in the aerosol mass spectrometer (AMS), is the ability to
analyze both non-refractory (e.g., organics, ammonium ni-
trate) and refractory (e.g., mineral dust, soot) components of
individual atmospheric aerosol particles (Pratt and Prather,
2011). However, a deeper investigation is required in order
to promote the laser ionization technique as a suitable ex-
perimental device for the elemental quantification of individ-
ual aerosol particles. The recently launched Laser Ablation
Aerosol Particle Time-of-Flight Mass Spectrometer (LAAP-
ToF-MS), based on laser desorption and ionization, provides
information on the aerodynamic diameter and chemical com-
position of individual aerosol particles. LAAP-ToF-MS is
intended for on-line and continuous measurement of atmo-
spheric particles with an analysis time in the order of mil-
liseconds per particle.
Here we present a laboratory-based study of the LAAP-
ToF-MS instrument performance and a novel approach to de-
veloping an analytical methodology for continuous monitor-
ing of particle size distribution and their composition using
this instrument. It will allow both qualitative information on
single particles and quantitative information about ambient
particle ensembles to be obtained simultaneously.
2 Experimental
2.1 Description of the LAAP-ToF-MS instrument
The LAAP-ToF-MS instrument (AeroMegt, GmbH) features
an aerodynamic particle lens inlet, a particle-sizing region
using two scattering lasers, a bipolar time-of-flight mass
spectrometer and an excimer laser as ablation/ionization
laser. The particle inlet is comprised of an aerodynamic lens
with a transmission for particles with an aerodynamic diam-
eter between 80 and 600 nm. The working principle of the
LAAP-ToF-MS is shown in Fig. 1a.
The aerosol particles leave the differential pumping stages
(inlet) and enter into the detection region where they pass
through the region irradiated with light (λ= 405 nm), emit-
ted by two continuous wave (cw) lasers (scattering lasers)
with a power range between 100 and 450 mW, facilitating
particle sizing by light scattering. The flight path between the
two laser beams has a length of 11.5 cm. The time between
the two scattering events, i.e. the particle’s time of flight, is
recorded and used to calculate the aerodynamic particle size.
In addition, the second scattering event triggers the excimer
laser that fires and ablates the drifting particle in its path. The
ionization laser is a 193 nm ArF* excimer laser (GAM Laser
Inc.) with a maximum energy of 10 mJ per pulse (pulse dura-
tion ∼ 10 ns) enabling ablation of single particles every 4 µs.
The LAAP-ToF-MS is operational in three modes of fast
triggering. (i) The first mode provides information about the
particle size and chemical composition of individual aerosol
particles; in this mode the excimer laser is triggered by two
consecutive light scattering events in both diodes. (ii) In the
second mode the excimer laser is triggered by the second
scattering laser only, allowing the calculation of high parti-
cle hit rates, without providing size information on the parti-
cles. (iii) In the third mode the excimer laser is fired without
a trigger pulse at constant frequency in the range between
1 and 100 Hz and particles will be ablated arbitrarily if they
happen to be in the path of the laser beam. In this study, only
the performance of the first mode will be described. In this
mode it is possible to study the chemical composition as a
function of the particle size (Buzea et al., 2007).
After ablation, the charged ions are extracted into a bi-
polar time-of-flight mass spectrometer (Tofwerk, BTOF)
with a resolving power of m/1m≥ 600 FWHM (Full Width
at Half Maximum) for both ion polarities. The ions are ex-
tracted into their corresponding flight region (positive or
negative ions) and detected by microchannel plate detec-
tors (MCPs). Positive and negative ions are detected inde-
pendently; both mass spectra (positive and negative), as well
as the related scattering signals, are recorded together and
can be further analyzed.
2.2 Experimental setup
2.2.1 Laboratory experiments
Two types of particles were used for laboratory experiments,
spherical particles of Polystyrene Latex beads (PSL, Duke
Scientific Corp) with a factor shape equal to 1 and a density
of 1.05 g mL−1, and ammonium nitrate particles (ACROS or-
ganics) with a factor shape equal to 0.8 and a density of
1.7 g mL−1.These particles were generated by an atomizer
Atmos. Meas. Tech., 9, 1947–1959, 2016 www.atmos-meas-tech.net/9/1947/2016/
R. Gemayel et al.: Laser ablation aerosol particle time-of-flight mass spectrometry 1949
Figure 1. (a) Schematic diagram of the working principle of LAAP-ToF-MS, (b–c) Experimental configuration aimed to investigate the
influence of particle density, size effect and detection efficiency, and (c–d) Experimental configuration for aerosol particle measurement.
(model 3076, TSI, US). A diffusion dryer (model 3306, TSI,
US) was used to decrease the humidity so it does not affect
the hit rate and the particle size. The number concentration is
regulated by a concentration controller. To control the num-
ber concentration in the sample flow, the particle flow is split
into two, one flow path passing through the particle filter
while the second one goes through a normal tube. The two
flows are then merged at the outlet of the concentration con-
troller. By increasing the fraction of the flow passing through
the filter, the particle number concentration decreases.
The experimental configurations were designed to investi-
gate the instrument’s performance in the first mode of oper-
ation, with particle sizing. The first outline (Fig. 1b–c) was
employed to study the repeatability, the size calibration and
the effect of the particle size and the particle number con-
centration on the hit rate of the excimer laser (HR) and the
scattering efficiency of the scattering lasers (E). The dif-
ferential mobility analyzer (DMA 3081, TSI, US, impactor
size 0.071 cm, sample flow= 0.3 L min−1 (liter per minute),
sheath flow= 3.0 L min−1) was placed downstream of the
particle generation assembly and was set to select particles
in the required size range between 15 and 773 nm. The sized
particle stream leaving the DMA was split between a conden-
sation particle counter (CPC 3776, TSI) (F1 = 0.3 L min−1)
and the LAAP-ToF-MS (F2 = F3 = 0.08 L min−1), to obtain
independent measurements of the number of particles per
second and the particles’ number in the DMA-selected size
range, respectively, allowing calculation of the scattering ef-
ficiency and the detection rate.
2.2.2 Ambient measurements
The second configuration (Fig. 1d–c) was used for the mea-
surement of atmospheric particles. This second configuration
was designed to assess the potential effect of chemical com-
position on the hit rate and the scattering efficiency of real
particles and to assess the effect of the number concentration.
The chemical composition, particle size and the number evo-
lution of the ambient particles were measured continuously
by the LAAP-ToF-MS and an optical particle counter (OPC
1.109, Grimm, Germany).
3 Results and discussion
3.1 Detection efficiency
The first step in the analysis of the processed raw data is to
evaluate the detection efficiency and to test the repeatability
of the performed analysis. To this end we need to introduce
three different concepts of instrumental efficiency. The detec-
tion efficiency (DE) is defined as a product of the scattering
efficiency of the laser diodes (E) and the ionization efficiency
of the excimer laser, also known as hit rate (HR):
DE (%)= E×HR. (1)
The scattering efficiency of the laser diodes is defined as
the ratio between the frequency of the detected particles by
LAAP-ToF-MS and the number of particles detected by the
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1950 R. Gemayel et al.: Laser ablation aerosol particle time-of-flight mass spectrometry
CPC per unit of time:
E (%)=N × 100
c×U × t, (2)
where N is the number of particles detected by the laser
diodes of the LAAP-ToF-MS, c is the number concentration
[cm−3], U is the aerosol sampling flow rate [80 mL min−1]
and t is the time [minutes]. The hit rate represents the ra-
tio between the number of ablated/ionized particles and the
number of particles detected by the laser diodes:
HR (%)=Ni × 100
N, (3)
where Ni is the number of ablated particles by the excimer
laser, which are, in turn, measured by ToF-MS yielding the
associated mass spectra. The hit rate depends on the thresh-
old setting discriminating between the useful spectra and to-
tal spectra. The intensities of real spectra depend on how suc-
cessful the laser ablation is. Laser ablation is a process that is
hard to replicate because the particles are randomly ablated.
Thus, each particle ablation event is different: (1) particles
can be completely missed by the laser pulse, (2) there can be
partially ablated particles, and there can be (3) completely
ablated particles. The threshold is considered as a better
discriminant than other measures, such as spectral variance
around the baseline, because it allows low-intensity spectra
to be included in the useful category, while excluding spectra
without distinct peaks, but which may have noisy baselines.
Inspection of excluded spectra is necessary for assessing the
correct value of the discriminant, i.e. the threshold.
3.2 Repeatability
Laboratory experiments
The repeatability of the LAAP-ToF-MS instrument was
tested by continuous analysis of polystyrene latex (PSL) par-
ticles with a diameter of 450 nm and a number concentration
of 39± 5 particles cm−3. The period of repeatability tests is
limited by the use of silica gel for particles drying which is
efficient for 53 h maximum. The repeatability test for both
the scattering efficiency and the hit rate, during the total time
period of 53 h, is shown in Fig. 2.
Every point in this figure corresponds to an average of de-
tected particles during a period of 3 min, which is a minimum
time interval necessary to attain sufficient number of detected
particles. The scattering efficiency varies between 0.6 and
1.9 % with an average of 1.1 %. The relative standard devia-
tion (RSD) is 17 % over the entire period of 53 h of analysis.
The hit rate exhibits good repeatability with an average value
of 63 % and the RSD is 18 %. The scattering efficiency may
decrease due to larger particles passing through the critical
orifice leading to a lower flow rate in the inlet. The argon
fluoride gas lifetime is another important parameter which
influences the hit rate. To test this parameter we generated
Figure 2. Repeatability of the scattering efficiency (E) and the hit
rate (HR), during a time period of 53 h.
Figure 3. The influence of the ArF gas life time on the evolution of
the laser hit rate (HR) over the time.
PSL particles with a diameter of 450 nm seven times for few
minutes each and then measured the hit rate. The first mea-
surement was made immediately after refilling the excimer
laser and the time difference between the first measurement
and the last one was 12 days. Figure 3 shows the variation
of the hit rate with time. During the first week the hit rate
is considered constant, and from the eighth day it begins to
decrease. Four weeks after refilling the excimer laser the hit
rate has dropped down to zero upon daily use of the laser.
According to the Laser Gam Ex5 specifications, laser energy
drops to 50 % after a shelf life of 12 days or after 12 million
pulses of ArF excimer laser. It seems that the shelf life is the
limiting factor when using the laser in association with sin-
gle particle mass spectrometer, at least in the diode trigger
modes. Therefore, the data shown in Fig. 3 correspond only
to the first 12 days.
The alignments of the scattering laser, aerodynamics
lenses and the ionization laser are done manually. Therefore,
the average of scattering efficiency and the hit rate are not
the same as above for the experiments discussed in the rest
of this article. However, the values of repeatability are ex-
pressed as relative standard deviation, which is not based on
Atmos. Meas. Tech., 9, 1947–1959, 2016 www.atmos-meas-tech.net/9/1947/2016/
R. Gemayel et al.: Laser ablation aerosol particle time-of-flight mass spectrometry 1951
Figure 4. The total particle number concentration detected by LAAP-ToF-MS and OPC as a function of time; indicated are peaks corre-
sponding to smoking events (a and b) and to generation of TiO2 (c). The total PM2.5 results according to Air PACA are depicted in green.
the alignment. Therefore, for a good repeatability of the scat-
tering efficiency during a field campaign it is important to fil-
ter out large particles to maintain a constant flow in the inlet
for as long as possible, while for a good repeatability of the
hit rate it is strongly recommended that the excimer laser is
refilled once a week.
3.3 Ambient measurements
Ambient aerosol measurements were performed on the cam-
pus of Aix-Marseille University, situated in the city center of
Marseille, France. The ambient air was simultaneously sam-
pled by LAAP-ToF-MS and OPC for a period of 6 days. A to-
tal of 62 813 bipolar mass spectra of single particles with dif-
ferent sizes were recorded, among which 36433 spectra were
useful.This corresponded to a hit rate of 58 %. The number of
particles detected every 5 min by OPC, in the range between
265 nm and 3 µm (aerodynamic diameter), is shown in Fig. 4.
The total number of particles in the range between 200 nm
and 3 µm (aerodynamic diameter), detected every 5 min by
LAAP-ToF-MS is also depicted.
As shown in Fig. 4, there are three peak events detected
during this monitoring campaign. Two of these particle num-
ber concentration spikes (a and b), with maxima of 510.9
and 607.5 particles cm−3, were detected on 7 January 2015 at
10:17 a.m. and 02:27 p.m., respectively, correspond to smok-
ing events near the building. The third peak (c), detected on
9 January 2015 is related to the generation of TiO2 parti-
cles that we intentionally introduced to the ambient air. Al-
though, these phenomena only lasted a few minutes they
were detected by LAAP-ToF-MS. As can be observed from
Fig. 4 there is a strong agreement between the three peaks
detected by OPC and LAAP-ToF-MS. Figure 4 also shows
good agreement between the particle number concentrations
detected by LAAP-ToF-MS and the results obtained by the
air monitoring station (Air PACA) which is located at 1.6 km
distance from our sampling site. The results of Air PACA
shown in Fig. 4 correspond to the particle mass concentra-
tions of PM2.5. The absence of the three peaks detected by
LAAP-ToF-MS is logical since these peaks were caused by
events happening on the sampling site, as described above.
The LAAP-ToF-MS measurements permit the identifica-
tion and the monitoring of several types of ions. Figure 10
shows the standard deviation of all superimposed positive
and negative ions mass spectra.
The negative ion mass spectra contain peaks associated
with elemental carbon (24C−2 ), nitrate (46NO−2 ) and sul-
fate (97HSO−4 ). The presence of cyanide (26CN−), (17OH−),
(35Cl−) can also be observed in Fig. 5. In the positive ion
spectra, the identified ion peaks are associated with elemen-
tal carbon (12C+1 , 24C+2 , 36C+3 ) and nitrate (30NO+). Also
potassium (39K+) and to a lesser extent sodium (23Na+) and
silicon (28Si+) are present. The two specific ions related to
TiO2 (48Ti+ and 64TiO+) were also observed. Other metal
ions such as lead, cerium and tin were also detected. The
source apportionment of these elements is outside the scope
of this article.
3.4 Parameters influencing the detection efficiency
The detection efficiency of the particles can be influenced
by the particle number concentration in the sample flow, the
size of the particles and the chemical composition which
can vary during the analysis. For this purpose, five different
number concentrations of ferric sulfate particles ranging be-
tween 50 and 1200 particles cm−3 were analyzed to evaluate
the number concentration effect. On the other hand five dif-
ferent sizes of PSL particles (350, 450, 500, 600, 700 nm)
were analyzed at the same particle number concentration,
20 particles cm−3, to assess the particle size effect. Several
repeat particle analyses were performed for each particle size
and particle number concentration.
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1952 R. Gemayel et al.: Laser ablation aerosol particle time-of-flight mass spectrometry
Figure 5. The standard deviation of all positive and negative ion mass spectra.
3.4.1 Size effect
Laboratory experiments
To test the influence of particle size on the efficiency of
the scattering lasers and the hit rate of the excimer laser,
five different sizes of PSL particles (350, 450, 500, 600,
700 nm) were analyzed at constant particle concentration of
20 particles cm−3. For this particle concentration, particles
smaller than 350 nm are undetectable. The RSD for each par-
ticle size, obtained from several replicate analyses, was com-
pared to the coefficient of variation corresponding to differ-
ent particle sizes. The particle size influences both the laser
scattering efficiency and the hit rate, and therefore the detec-
tion efficiency of LAAP-ToF-MS (Fig. 6), as well.
Figure 6 shows that the hit rate decreases with the parti-
cle diameter, from 93 to 83 % when the diameter decreases
from 600 to 350 nm. This behavior can be explained by the
fact that smaller particles drift with higher velocity. Thus, the
ions generated by the ionization laser have a higher kinetic
energy resulting in aberrations (Murphy, 2007). A maximum
efficiency of 2.5 % for the laser scattering diodes was ob-
served for particles with a diameter of 450 nm and a lower
efficiency for smaller particles. When the size of the individ-
ual particles becomes equivalent to or greater than the wave-
length of the laser (λ= 403 nm), the scattering becomes a
complex function with maxima and minima with respect to
the incident angle according to Mie theory (Finlayson-Pitts
and Pitts, 2000). As the diameter of the particle drops below
the wavelength of the scattering laser the scatter intensity de-
creases rapidly, inversely proportional to the sixth power of
the particle diameter (1/d6).
Figure 6. The scattering efficiency and the hit rate as a function of
the size of various PSL particles (350, 450, 500, 600, and 700 nm)
at a particle number concentration of 20 cm−3. The SPLAT and
SPLAM scattering detection efficiency results are given for com-
parison purpose.
The scattering efficiency decreases again for particles with
a dva diameter greater than 600 nm as only the particles in the
range between 80 and 600 nm are transmitted at 100 % by the
aerodynamic lenses.
A comparison between the scattering efficiencies of
LAAP-ToF-MS, the single particle laser ablation mass spec-
trometer (SPLAM) (Gaie-Levrel et al., 2012) and the sin-
Atmos. Meas. Tech., 9, 1947–1959, 2016 www.atmos-meas-tech.net/9/1947/2016/
R. Gemayel et al.: Laser ablation aerosol particle time-of-flight mass spectrometry 1953
gle particle laser ablation time-of-flight mass spectrome-
ter (SPLAT) (Zelenyuk and Imre, 2005) has been under-
taken (Fig. 6). The scattering efficiency of SPLAT de-
creases slightly for particles higher than 300 nm compared to
SPLAM or LAAP-ToF-MS. The scattering efficiency shows
the same behavior for LAAP-ToF-MS and SPLAM which
can be ascribed to the same operating wavelengths of the
scattering lasers (λ= 405 nm for SPLAM). However, the
scattering efficiency of SPLAM is much higher than that
of LAAP-ToF-MS, which can be explained by the much
smaller distance (dd) between the two scattering lasers within
SPLAM, i.e. 4.1 vs. 11.5 cm for LAAP-ToF-MS. Another
advantage of SPLAM compared to the LAAP-ToF-MS is
the higher value of Cmax which is ascribed to the small dd.
The distance between the two scattering lasers influences
the Cmax for a particle size of 350 nm and a velocity of
103 m s−1, the Cmax of LAAP-ToF-MS is 618 particles cm−3
whereas the Cmax of SPLAM for the same particle size and
a velocity of 100 m s−1 is 1.7× 103 particles cm−3. The ra-
tio between the dd of SPLAM and LAAP-ToF-MS is 2.87
and is similar to the ratio between the Cmax of SPLAM and
LAAP-ToF-MS (2.75), which explains that divergence of the
particle beam increases with dd and is more pronounced for
smaller particle sizes. In comparison to SPLAM, which uses
ionization laser at λ= 248 nm, the ablation of the particles
by LAAP-ToF-MS occurs at 193 nm which means that even
metals can be ionized. A big advantage of LAAP-ToF-MS
compared to SPLAM or SPLAT is the much higher hit rate.
For LAAP-ToF-MS the effective hit rate is 90 % for PSL par-
ticles and 58 % for atmospheric particles, while the hit rate of
SPLAT is only 8 % for atmospheric particles. Also, LAAP-
ToF-MS is an easily transportable tool for fast field deploy-
ment.
Finally, a comparison was carried out with another similar
instrument named Aerosol Time of Flight Mass Spectrom-
eter (ATOFMS) (Gard et al., 1997). This instrument oper-
ates at 266 nm unlike the LAAP-TOF-MS (λ= 193 nm). The
lower wavelength of the ionization laser enables the analysis
of trace metals. There are few papers in the literature refer-
ring to the development of ATOFMS associated with detec-
tion of different size of particles (Allen et al., 2000; Su et al.,
2004; Zauscher et al., 2011). For example, the detection effi-
ciency of ATOFMS is highest for the ambient particles with
diameter of 1.8 µm and decreases for about 3 orders of mag-
nitude for the lowest size that is 320 nm (Allen et al., 2000).
Su et al. (2004) reported that ATOFMS is able to detect small
size particles ranging between 70 and 300 nm with detection
efficiency varying between 0.3 and 44.5 %.
In any case, it should be noted that size has an impact on
the detection efficiency as we mentioned above.
Ambient measurements
We assessed the size effect of ambient aerosols on the hit rate
and on the scattering efficiency. For each size in the range
between 10 nm and 2.5 µm (aerodynamic diameter) we are
showing (Fig. 7) the total number of particles detected by the
LAAP-ToF-MS during the measurements by the scattering
lasers and also the total number of ionized particles during
the measurements.
The optimum particle size for detection is in the range be-
tween 400 and 600 nm (aerodynamic diameter), in the same
range as the wavelength of ionization (λ= 403 nm). The
Fig. 7b shows the time evolution of the particle concentra-
tion. It can be seen that in the ambient air the maximum
particle number concentration corresponds to the lowest size
range (dva < 300 nm). The comparison between the results of
the Fig. 7a and the results of the Fig. 7b confirm the conclu-
sions from laboratory tests that the scattering efficiency is af-
fected by the size of particles and its maximum is influenced
according to the Mie theory.
In addition, Fig. 7a shows that the hit rate for ambient
aerosol as function of the size range is different from the lab-
oratory results. This difference can be ascribed to the effect
of chemical composition which is detailed in Sect. 3.4.3.
Figure 7c shows the evolution of the number of spectra
in each size range every 5 min during the measurements.
Since the scattering efficiency and the hit rate are affected
by the particle size, so is the detection efficiency (Fig. 7c).
Most of the usable spectra are in the range between 400 and
500 nm. The effect of particle size is overcome by cluster-
ing the spectra obtained for each size range and multiply-
ing the number of ionized particle by the detection efficiency
(D%= E×HR) corresponding to each size range.
3.4.2 Effect of the distance between the two scattering
laser
Laboratory experiments
We investigated the transmission efficiency between the first
and the second scattering laser, considering that the two laser
diodes have the same characteristics. However, the first scat-
tering laser exhibits a much higher efficiency (Ed1) than the
second scattering laser (Ed2). This observation is a conse-
quence of the divergence of the particles between the two
laser diodes. In order to understand the magnitude of the par-
ticle divergence we researched into the relationship between
the ratio of scattering efficiencies Ed2/Ed1 (%) and the par-
ticle size. Figure 8 displays a parabolic dependency of the
ratio of the scattering efficiencies with the size of the PSL
particles generated, indicating that velocity indeed plays an
important role.
Smaller particles with a diameter of 350 nm exhibit higher
velocities and diverge much more than bigger particles with a
size of 600 nm. This curve also explains the lower scattering
efficiency of particles with a diameter of 350 nm displayed in
Fig. 8.
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1954 R. Gemayel et al.: Laser ablation aerosol particle time-of-flight mass spectrometry
Figure 7. (a) Total number of particles detected and ionized during the ambient measurements in different size range and the hit rate
corresponding to each size range (aerodynamic diameter). (b) The evolution of the particle number concentration of the ambient aerosol
detected by the OPC during the measurements for different size range depicted between 275 and 2500 nm (aerodynamic diameter). (c) The
evolution of the number of particles ionized during the ambient measurements in different size range depicted between 10 and 2500 nm.
Figure 8. The ratio Ed2/Ed1 (%) as a function of the PSL particle
size.
In this study there are no information about the values of
detection limit in number concentration for each particle size,
because this limit is different for each type of particle.
Liu et al. (1995) have demonstrated that the morphology
of the particles is a very important parameter that influences
the divergence of particles during their drift between the two
scattering lasers. In fact, the divergence of the particles in-
creases for non-spherical particles implying a reduction of
the scattering efficiency of the laser diodes.
3.4.3 Chemical composition
Laboratory experiments
The ionization efficiency of the excimer laser depends on
the chemical composition of the particles (Pratt and Prather,
2011). Experiments were carried out with two types of par-
ticles containing ammonium nitrate and ammonium sulfate
in order to assess the effect of chemical composition on
LAAP-ToF-MS performance. Although, both particles have
the same density (1.74± 0.03 g cm−3) and the same shape
factor (0.8), the hit rate is completely different. Because sul-
fate resists ionization (Kane and Johnston, 2001), the hit rate
decreases from 60 % for the ammonium nitrate particles to
21 % for the ammonium sulfate particles. The hit rate also
strongly depends on the alignment of the ionization laser
and on the delay time. A change in the chemical particle
composition induces a change in the refractive index. Yoo et
al. (1996) evaluated the influence of the refractive index on
the scattering efficiency of laser diodes. The higher the re-
fractive index, the smaller the particles that can be measured.
Moffet and Prather (2005) developed a method to calibrate
the light scattering signal collected from individual particles
using the Mie theory to calculate the partial scattering cross-
section as a function of the particle diameter. The particle
density was used to fit the partial scattering cross-section to
the Mie theory (Moffet and Prather, 2005).
Atmos. Meas. Tech., 9, 1947–1959, 2016 www.atmos-meas-tech.net/9/1947/2016/
R. Gemayel et al.: Laser ablation aerosol particle time-of-flight mass spectrometry 1955
Figure 9. (a) Different clusters of particles and their evolution during the measurements in different size range between 10 and 2500 nm. (b)
The standard deviation of the total hit rate calculated every 5 min during the measurements for each size range.
Ambient measurements
The complete set of spectra can be clustered using the soft-
ware MATLAB version 2013b into different chemical classes
of particles.
Figure 9a illustrates four of these clusters and their repar-
tition every 5 min in different size range. These clusters were
chosen as example to show different kind of inorganics par-
ticles, and one cluster with major carbonaceous ions. The in-
organic particles are those containing sulfate and nitrate that
are considered as secondary particles and particles contain-
ing TiO2 that are rather considered as primary particle (Del-
mas et al., 2005). It can be observed that nitrosium ion NO+
(m/z= 30) is abundant in the first cluster and potassium ion
K+ (m/z= 39) is abundant in the second cluster. The third
cluster represents particles with high signals of carbon, and
in the fourth cluster characteristic peak of carbon C+ (m/z=
12), C+2 (m/z= 24), C+3 (m/z= 36) dominate.
Every cluster has its own repartition, which is defined as
a number of particles detected every 5 min in different size
range. Thus, the chemical composition of the particles de-
tected during the measurements is not constant. To show the
effect of chemical composition on the hit rate we calculated
the hit rate of particles with different size range every 5 min
during the entire time of the measurements. Then we calcu-
lated the RSD of the hit rate for each size range. The RSD
varies between 51 % for the aerodynamic size range between
400 and 500 nm to 96 % for aerodynamic size range between
800 and 1000 nm (Fig. 9b). Comparing the RSD of ambi-
ent particles to the RSD calculated of spherical PSL particles
during the laboratory tests (Sect. 3.2, repeatability 18 %), it
can be concluded that chemical composition of particles af-
fects the hit rate.
The effect of chemical composition on the hit rate was as-
sessed for particles ranging between 400 and 500 nm (aero-
dynamic diameter). Figure 10a shows the evolution of the
scattering efficiency and the hit rate for the detected particles
between 400 and 500 nm (aerodynamic diameter).
It can be seen that the hit rate and the scattering efficiency
are not constant all the time. As was already seen for a single
type of particles the instrument exhibits good repeatability.
Therefore the variation in HR (%) and E (%) is mainly the
consequence of the variation of the chemical composition. In
Fig. 10a and c the variation of the number of three types of
particles is represented. The first type (Fig. 10b) represents
the particles having an aerodynamic size between 400 and
500 nm and containing sulfate (cluster 1 and 2). The second
type represents the particles having an aerodynamic size be-
tween 400 and 500 nm and containing a TiO2 (cluster 3). The
third type (Fig. 10c) (cluster 4) represents the carbonaceous
particles having a size between 400 and 500 nm. The increase
and decrease of the percentage of particles containing sulfate
is illustrated by the peak and trough (points G) depicted in
Fig. 10b. The points G′ depicted in Fig. 10a correspond to a
decrease of hit rate according to the peak of sulfate and an
increase of hit rate caused by the decrease of the percentage
of sulfate. The point A in Fig. 10b shows the highest percent-
age of sulfate in parallel to a low hit rate shown in Fig. 10a.
The point S which corresponds to a maximum concentration
of TiO2 (Fig. 10b) shows a very low value of scattering effi-
ciency and hit rate (Fig. 10a). Regarding the points P , R and
www.atmos-meas-tech.net/9/1947/2016/ Atmos. Meas. Tech., 9, 1947–1959, 2016
1956 R. Gemayel et al.: Laser ablation aerosol particle time-of-flight mass spectrometry
Figure 10. (a) The scattering laser E (%) and the evolution of the hit rate HR (%) of the LAAP-ToF-MS for particles having a size between
400 and 500 nm. (b) The evolution of the sulphate particles and the particles containing TiO2. (c) The evolution of the elemental carbon
particles. A, G and G′ represent the influence of the percentage of sulphate containing particles on the HR (%). S corresponds to the
maximum concentration of TiO2 and very low values of scattering efficiency and hit rate. P ,R and T represent the influence of the percentage
of carbonaceous particles on the scattering efficiency.
T the number of carbonaceous particles decreases while the
number of TiO2 particles increases. For these three points the
scattering efficiency decreases, as well. The evolution of the
carbonaceous particles before and after S exhibits a similar
behavior as the hit rate. Despite the effect that other particles
could induce on these parameters, the comparison made in
Fig. 10 emphasizes the importance of chemical composition
toward the hit rate and the scattering efficiency.
Therefore, a simple separation by size range and a correc-
tion of the detection efficiency according to the size can no
longer lead to the real concentration number because of the
variation of the chemical composition. Thus, the average of
the detection efficiency calculated for each size range is no
longer adequate for a time interval of few minutes. Therefore,
it is necessary to have a particle counter (like an OPC) to cal-
culate the detection efficiency (Dn, t ) for each size range for
every time interval. On the other hand, the total amount of
particles must be separated in different classes (Ci) based on
their chemical composition. These classes must be separated
in different size ranges (Ci, n). Every Ci, n, according to its
distribution during the time, must be multiplied by its corre-
spondingDn, t . The description of this method is out of scope
of this article and therefore will be detailed and validated by
comparison to another instrument elsewhere.
Size calibration
Ambient measurements showed that a significant amount of
particles could be related to particles with a diameter less
than 350 nm, which is not the case for experiments with the
spherical PSL particles during the calibration of the instru-
ment. This can be explained by the fact that particles in ambi-
ent air have different optical characteristics, enabling them to
scatter the light more efficiently at the scattering wavelength
used in this instrument (λ= 405 nm). Therefore, in order to
precisely determine the diameter of the particles we carried
out measurements related to the size calibration of the parti-
cles.
When a particle drifts through the particle-time-of-flight
(P-ToF) chamber, it crosses the beam of two light scatter-
ing lasers. Upon passing the first laser beam, the scattered
light from the particle is detected by the first photomultiplier
tube (PMT). As explained above in the description of LAAP-
ToF-MS, the flight time of an individual particle between the
first and second scattering lasers is used to determine its ve-
locity and associated vacuum-aerodynamic diameter. For the
given beam separation distance of 11.5 cm between the two
scatterings lasers the particle velocity was determined and
plotted against the aerodynamic particle diameter (Fig. 11).
Atmos. Meas. Tech., 9, 1947–1959, 2016 www.atmos-meas-tech.net/9/1947/2016/
R. Gemayel et al.: Laser ablation aerosol particle time-of-flight mass spectrometry 1957
Figure 11. Plot of aerodynamic particle size versus particle velocity
for (a) PSL particles and (b) ammonium nitrate particles.
Figure 11 shows the calibration curve for aerodynamic
particle sizing measurements carried out for five certified
sizes of PSL particles (a) and five different sizes of ammo-
nium nitrate particles (b).
The experimental data were fitted with a first order expo-
nential decay curve. The smallest PSL particles that can be
precisely size-calibrated have a diameter of 350 nm. How-
ever, the fitting equation depicted in Fig. 11 can serve to
roughly estimate the size of atmospheric particles with an
aerodynamic diameter smaller than 350 nm.
3.4.4 Particle number concentration effect
Laboratory experiments
Prior to study the effect of number concentration, an upper
limit of the particle number concentration (Cmax) has been
determined for each size to ensure that below this limit only a
single particle is present in the space between the two scatter-
ing lasers. The obtained results presented in Fig. 12 indicate
that Cmax is linear and inversely proportional to the particle
size.
For a particle size of 350 nm, which is the smallest par-
ticle size that has been tested, Cmax is ≈ 618 particles cm−3.
For higher particle number concentrations, more particles are
present in the space between the two scattering lasers which
indicates that smaller particulate matter with d < 200 nm can
be detected but the obtained information corresponds to two
different particles detected in very small frame of time. In
other words, the spectrum obtained relates to a single, real
particle, but the size information does not. Hence, the E (%)
should decrease because the data of one single particle is
Figure 12. Variation of Cmax for particles with different aerody-
namic diameters.
recorded instead of two. In order to study the effect of a
concentration higher than Cmax on the E (%), ferric sul-
fate particles (450 nm) were generated at 5 different con-
centrations between 50 and 1200 particles cm−3. The higher
level 1200 was chosen according to the value of Cmax found
at 562 particles cm−3 for particles with diameter of 450 nm.
The influence of particle concentration on the detection ef-
ficiency was assessed by comparison of the obtained RSD
values based on at least three independent measurements.
Concerning the scattering efficiency E (%), it was ex-
pected that it decreases, but the RSD between the different
concentrations is lower than the RSD between the repeti-
tions for the same concentration, so the E (%) is considered
constant. To study the effect of concentration number higher
than Cmax on the detection of particles lower than 200 nm to
which the scattering lasers are blind, the percentage of these
particles for the different concentration numbers studied was
assessed (Fig. 13). Once the concentration is higher than the
Cmax, 562 particles cm−3, the percentage of the particles with
size lower than 200 nm increases from 1 % for a concentra-
tion number of 40 particles cm−3 to 19 % for a concentra-
tion number of 612 particles cm−3. This means that the de-
tected particle with diameter lower than 200 nm corresponds
to the detection of two different particles by the two scatter-
ing lasers.
Ambient measurements
The detected particles in the range between 250 and 350 nm
(aerodynamic diameter) could be the result of two phenom-
ena. The first one is the presence of a total concentration
number higher than the Cmax for all the particle sizes and
the second one is the increase of the refraction index of the
particles. A comparison of the results obtained by the OPC
and the LAAP-ToF-MS, that has been undertaken for the
particles ranging between 250 and 350 nm shows the reason
why these particles were detected. The comparison of the re-
www.atmos-meas-tech.net/9/1947/2016/ Atmos. Meas. Tech., 9, 1947–1959, 2016
1958 R. Gemayel et al.: Laser ablation aerosol particle time-of-flight mass spectrometry
Figure 13. The hit rate and the scattering efficiency of 450 nm ferric
sulfate particles as a function of the particle number concentration
and the percentage of the particles having a size lower than 200 nm
for different concentrations of generated particles.
sults is depicted in Fig. 14a where a similar evolution of the
number of particles is shown for the two types of measure-
ments. The figure indicates that the particles between 250
and 350 nm detected by the LAAP-ToF-MS are not a con-
sequence of the total concentration of particles which was
higher than the Cmax during the 6 days of measurements.
Considering that the scattering laser is blind with respect
to the particles with dva<200 nm and that the aerodynamic
lenses cannot transmit particles with dva < 80 nm, the effect
of Cmax was evaluated as shown in Fig. 14b. Particles having
an aerodynamic diameter between 0 and 80 and 0 and 200 nm
were detected mainly when the number concentration of par-
ticles increased (Fig. 14b).
4 Conclusions
A recently developed LAAP-ToF-MS instrument has been
calibrated and characterized.
In this work the performance of LAAP-TOF-MS has
been characterized on standard spherical particles under con-
trolled laboratory conditions and on ambient particles.
Prolonged on-line measurements revealed that the detec-
tion efficiency of LAAP-ToF-MS and the hit rate exhibits
good repeatability with RSD of 17 and 18 %, respectively.
A comparison between the detection efficiency of LAAP-
ToF-MS and the scattering efficiency of single particle laser
ablation mass spectrometer (SPLAM) showed that the detec-
tion efficiency as a function of particle size is very similar.
A maximum detection efficiency of 2.5 % was observed
for particles with a diameter of 450 nm with a decreasing ef-
ficiency towards smaller sized particles. Therefore, to further
increase the accuracy of the data it is essential to improve the
detection efficiency for smaller particle sizes.
Figure 14. (a) The number of particles sizes between 200–300 nm
detected by the LAAP-ToF-MS every 50 min and the number con-
centration detected every 5 min by the OPC for the particles sizes
250–300 nm. (b) the number of particles having a size between 0–
200 and 0–80 nm detected by the LAAP-ToF-MS every 5 min.
Many parameters such as particle number concentration
in the sample flow, the size of the particles, and the chemical
composition, could change during a field campaign and af-
fect the detection efficiency of the LAAP-ToF-MS. For this
reason, the changing in the performances of this instrument
caused by the parameters cited above was studied using lab-
oratory and atmospheric particles. The temporal evolution of
the particles was validated during the ambient aerosol mea-
surements performed at the campus of Aix-Marseille Univer-
sity, situated in the city center of Marseille, France. The ob-
tained results are in good agreement with the data obtained
by optical particle counter and the PM2.5 data obtained by
the local air monitoring station. Also several metal ions were
detected during this field campaign such as lead, cerium, ti-
tanium and tin.
Therefore, LAAP-ToF-MS is a suitable instrument for on-
line monitoring of atmospheric particles that can provide
information on size distribution, number concentration and
chemical composition of the detected particles.
Acknowledgements. This work is a contribution to the LABEX
SERENADE (no. ANR-11-LABX-0064) funded by the “Investisse-
ments d’Avenir”, French Government program of the French Na-
tional Research Agency (ANR) through the A*Midex project (No.
ANR-11-IDEX-0001-02).
The authors gratefully acknowledge the support of this work by
French National Agency of Research within the ANR-10-EQPX-
39-01.
Edited by: P. Herckes
Atmos. Meas. Tech., 9, 1947–1959, 2016 www.atmos-meas-tech.net/9/1947/2016/
R. Gemayel et al.: Laser ablation aerosol particle time-of-flight mass spectrometry 1959
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